First of all, let me say that I really enjoyed playing your game and even managed to finish it. Which I cant say about most games I've played so far.
The movement feels great and I liked the handpainted art style a lot.
However, I would recommend a crosshair when making a shooter. Makes it a lot easier to hit enemies.
Also my biggest struggle was the stairs in the second level as they could be a bit buggy and in a perfect world your art style would have covered the whole game. A consistent art style is always really important for a clear visual style.
But I think you can really be proud at your game, definitely one of the better ones!